1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly, to a method of driving ferroelectric liquid crystal displays with improved contrast and brightness.
2. Discussion of the Related Art
Recently, liquid crystal displays (hereinafter abbreviated LCDs), which are low power consuming, low volume flat panel displays, have been replacing conventional cathode ray tubes in many applications. Liquid crystals having both liquid fluidity and optical crystal properties. LCDs operate by varying arrangement of liquid crystal using applied electric fields.
LCDs functionally include a liquid crystal panel and display drivers. The liquid crystal panel includes a lower (or array) substrate having pixel electrodes and thin film transistors arranged in a matrix, an upper (or common) substrate having a common electrode and color filter layers, and liquid crystal disposed between the upper and lower substrates.
There are different types of liquid crystals. For example, twisted nematic (TN) liquid crystals can be used to fabricate thin, low power, highly portable TN LCDs (twisted nematic liquid crystal displays). While beneficial in many respects, TN LCDs tend to have narrow viewing angles and relatively slow response times, thereby being rather unsuited for displaying high speed moving images.
Another type of liquid crystal is the ferroelectric liquid crystal (FLC). FLC has a property that enables in-plane switching of ferroelectric liquid crystal displays (FLCDs). In-plane switching can lead to improved viewing angles and faster response times as a result of spontaneous polarization. Therefore, FLCDs can have wide viewing angles and relatively fast response times. Thus, FLCDs are well suited for producing high speed moving images, thereby being a leading contender for next generation television sets.
FLCs themselves have various modes of operation, including the DHF (deformed helix FLC) mode, the SSFLC (surface stabilized FLC) mode, the AFLC (anti-ferroelectric LC) mode, the V type FLC mode (hereinafter abbreviated V mode), the Half-V type FLC mode (hereinafter abbreviated HV mode), and the like.
Much effort has gone into improving the FLC V mode because of it has advantages in gray realization and drive systems, and into improving the HV mode. Prototype V mode and HV mode LCDs have been reported.
HV mode is highly advantageous in that it enables a high contrast ratio, primarily owing to the superiority of its initial alignment state, suitability for active driving, and good temperature characteristics.
The initial alignment of the HV mode is established as follows. An electric field having a DC component that corresponds to a drive saturation voltage of the liquid crystal is applied between upper and lower electrodes during a phase transition (produced by temperature variation) of the liquid crystal from an initial N*state to an SmC* state. The applied electric field induces a spontaneous polarization direction along the applied electric field. Thereafter, unless disturbed (such as by an applied electric field), the liquid crystal molecules form a molecular arrangement along the spontaneous polarization direction induced by the initial alignment, thereby forming a uniform alignment state. For example, if the DC electric field used for initial alignment was negative (−), unless another potential is applied the liquid crystal molecules uniformly aligned in the directed induced by the negative (−) potential. Accordingly, when used in a display, the spontaneous polarization direction controls the alignment of the liquid crystal until a positive (+) electric field is applied. A negative (−) field has little or no effect on the liquid crystal. Thus, since only half of the possible electric fields significantly impact the liquid crystal alignment, the transmittance characteristic for applied data voltages is often called a Half-V (or HV as used herein) type FLC (cf. FIG. 3).
Meanwhile, the LCD display drivers include a central processor that outputs synchronous signals that are produced by processing video signals input from an external device. Additionally, a timing controller generates various timing signals required for image display from a synchronous signal output from the central processor. In particular, the timing controller produces a frame period, a basic time unit in which video data is displayed. Furthermore, a data drive part supplies data lines with output signals from a signal controller (based on outputs from the central processor), a gate drive part that sequentially applies scan voltages to the gate lines (based on outputs from the central processor), and a power supply that produces various required voltages. The On/off states of the thin film transistors (hereinafter abbreviated TFTs) depend on the voltages applied to the gate lines. In particular, a TFT channel opens when a TFT is turned on. Then, a pixel electrode is charged by the signal voltage on an associated data line. The result is video data displayed on the liquid crystal panel.
Specifically, the power supply produces a common voltage Vcom that is applied to the common electrode, and data drive part provides the liquid crystal panel with positive and negative video signals that represent an image that is to be produced by a pixel.
The positive and negative video signals are alternately applied as data voltages to the pixel (electrode), while a middle (between the positive and negative video signals) voltage, Vcom, is applied to the common electrode. This use of positive and negative video signals prevents the LC degradation that would result if only single polarity DC voltages were used.
The positive and negative video signals are not randomly applied. In practice there are a number of driving schemes that are used to prevent LC degradation. Those schemes include frame inversion, line inversion, column inversion, and dot inversion.
In dot inversion, both line and column inversions are used. This results in an improved image since flicker, an attribute of AC switching, tends to cancel out. In dot inversion the polarity of adjacent pixels differ.
FIG. 1 illustrates a drive waveform in a typical dot inversion method. Referring now to FIG. 1, the gate voltage Vscan determines the state of each TFT. For example, if a Vscan high voltage of 21V is applied to a gate, that gate is ‘ON’. If a Vscan low voltage of −5V is applied, that gate is ‘OFF’.
As shown, Vcom is a uniform DC waveform (and which is connected to the common electrode) and Vdata is a data voltage that is inverted, relative to Vcom, with the inversions occurring at a uniform rate according to a drive frequency that establishes frame periods. Vdata inversion compensates for the DC electric field accumulated in the previous frame so as to prevent ion accumulation in a liquid crystal (LC) cell, as well as LC degradation.
A nematic LC display driven by dot inversion has the transmittance T represented by the graphs shown in FIG. 2. As shown, the brightness (a function of transmittance) depends on the absolute magnitude of Vdata relative to Vcom. However, referring now to FIG. 3 and FIG. 4, the brightness of a HV mode FLC display depends on both the magnitude and on the direction of the applied electric field. In particular, one electric field polarity causes an increased brightness while the other polarity has little or no effect. This produces a brightness discontinuity. FIG. 3 and FIG. 4 shows a HV mode FLC display that responds only to a positive (+) polarity electric field. As shown, when driven by an alternating current Vdata signal the polarity of the electric field is inverted at a 1:1 ratio (Vcom as the reference voltage). Thus, an HV mode FLCD has a brightness that corresponds to half that of other LCDs.
Unfortunately, the method of driving a ferroelectric liquid crystal device according to the related art has problems. For example, when the initial aligmnent is achieved using a HV mode ferroelectric liquid crystal, that liquid crystals operate only with an electric field of one polarity. In particular, when the driving alternating current is inverted at a 1:1 ratio, bright and dark states alternate in each frame.
Hence, the equalized brightness is reduced to half that of a general nematic mode, which degrades image quality.